1. Field of the Invention
The present invention relates to an optical filter for obtaining a favorably out-of-focused image when taking photographs, as well as to a light amount adjusting apparatus and an image-taking apparatus to which such an optical filter is mounted.
2. Description of Related Art
In the past, technology has been disclosed related to optical filters for improving out-of-focused image, so-called blurred images which are not in focus and to cameras incorporating such optical filters. An example of such filters are apodization filters. Apodization filters are devised so that their transmission rate decreases with increasing distance from a center of the optical axis in directions perpendicular to the optical axis, and when such a filter is used, then the contours in a blurred image become smooth, and cross-eyed bokeh and ring bokeh are eased. Accordingly, in scenes in which the focus depth is small, such as when taking portraits or in macro photography, the background becomes a naturally blurred image with softened contours, attaining a high-quality image in which the focused main object looks attractive.
Regarding manufacturing methods and structures for filters attaining this effect, the following technology is known.
U.S. Pat. No. 3,843,235 discloses a structure of an apodization filter, in which a planar-concave lens made of light-absorbing glass and a convex-planar lens made of transparent glass are cemented together, and the optical power is set to substantially zero. Also disclosed are the optical characteristics when incorporating this apodization filter in an optical system. U.S. Pat. No. 3,843,235 also discloses a method for obtaining an apodization filter by applying a photosensitive resin on a transparent base and causing a coloring reaction in the light-sensitive resin through an optical mask, as well as a method for fabricating an apodization filter by a vacuum evaporation method on a transparent base. It is also mentioned that it is possible to provide a plurality of apodization filters having different transmission rate distributions, and to perform a light amount adjustment control that is optimal for each filter.
Japanese Patent Application Laid-Open No. H10 (1998)-268382 discloses a lens barrel to which an apodization filter can be mounted removably, and a camera in which aperture control is performed based on stored information corresponding to the type of the mounted filter.
Moreover, pages 74–77 of the April 1999 issue of “SHASHIN KOGYO” (Photo Industry), a monthly journal published by KK. SHASHIN KOGYO SHUPPANSHA, discloses a product application example of an apodization filter according to the technology disclosed in U.S. Pat. No. 3,843,235 and made by cementing together a planar-concave lens made of light-absorbing glass and a convex-planar lens made of transparent glass, and also discloses the transmission rate distribution characteristics of the apodization filter.
However, the following problems occur when the optical characteristics of the apodization filter are inappropriate:    (A) When a transmission rate at an outer periphery of a filter is too large, then an effect of smoothening the contours of blurred images is decreased.    (B) When a transmission rate at an outer periphery of a filter is too small, then a diameter of blurred images becomes small, and a three-dimensional impression and an impression of depth of an image are lost.    (C) When a shape of a transmission rate distribution is inappropriate, then the contours of blurred images become unnatural.    (D) When the average transmission rate of an overall effective filter portion is too small, then a transmitted light quantity is reduced, extending an exposure time and increasing an effect of camera shake.
Consequently, for the optical characteristics of an apodization filter, a shape of a transmission rate distribution is desired, at which an avoiding of the above-noted problems is well-balanced, but the conventional technology has the following problems:    (1) With a cemented glass filter as disclosed in U.S. Pat. No. 3,843,235 and on pages 74–77 of the April 1999 issue of “SHASHIN KOGYO,” that is, with the apodization filter made by cementing together the planar-concave lens made of light-absorbing glass and the convex-planar lens made of transparent glass, a shape of the transmission rate distribution is limited substantially to the Gauss distribution (also referred to as a “normal distribution” in stochastic theory), and it is difficult to obtain characteristics that provide a well-balanced solution of the problems (A) to (D) described above.    (2) There is no specific numeric limitation to the transmission rate distribution disclosed in U.S. Pat. No. 3,843,235, and there is also no explicit disclosure relating optical characteristics that should be provided in order to solve the problems (A) to (D).    (3) Japanese Patent Application Laid-Open No. H10 (1998)-268382 discloses a mechanism by which different filters can be mounted selectively in accordance with the image-taking objectives, but there is no disclosure with regard to a method for optimizing the effect of smoothening the contours of blurred images, or the filter characteristics for this.
However, the above-mentioned related technologies have the following disadvantages:
(1) U.S. Pat. No. 3,843,235 does not disclose anything about driving of the apodization filter with respect to an aperture through which a light flux is transmitted. It is mentioned that a plurality of apodization filters with different effective diameters and transmission rate distributions may be provided and used as appropriate, but there are no specific explanations at all regarding a changing mechanism or a method for exposure control. On the other hand, in the related technology disclosed in Japanese Patent Application Laid-Open No. H10 (1998)-268382, the camera operator must change the apodization filters manually, so that its handling properties are poor and there is the risk of missing out on good photo opportunities.
(2) With the apodization filter disclosed in U.S. Pat. No. 3,843,235, which is made by cementing together the planar-concave lens made of light-absorbing glass and the convex-planar lens made of transparent glass, the degree of freedom for the design of the transmission rate distribution is low, and moreover there is the problem that the image-forming optical system tends to become large, because a thickness of the filter has to be made large. Moreover, an advancing/retracting mechanism for on-off switching of the apodization filter (meaning the control of the advancing and retracting with respect to the aperture) is necessary, so that there was a need for a structure with which the image-taking apparatus into which the filter is incorporated does not become large. It should be noted that U.S. Pat. No. 3,843,235 contains statements which seem to indicate that it is possible to make the apodization filter thinner and lighter, that is, to manufacture the apodization filter by forming the photosensitive resin or the vapor-deposited film on the transparent base, but there is no description of any specific manufacturing method.
(3) Not only the above-described apodization filter made by cementing together the planar-concave lens made of light-absorbing glass and the convex-planar lens made of transparent glass, but also apodization filters made by forming the photosensitive resin or the vapor-deposited film on the transparent base, which can be expected to be made thinner and lighter in the future, are expensive in view of their workability and the facilities that are required to manufacture them.
On the other hand, in image-taking apparatuses for capturing still pictures or moving images by using an image-pickup system such as a silver-halide film or an image-pickup means such as a CCD, there are deficiencies with regard to not only the light amount adjusting function but also the focus adjustment function when trying to obtain high-quality images. However, in the related technologies, there is no disclosure regarding a method for focus adjustment when using an apodization filter.
Moreover, the Minolta STF 135 mm F2.8 [T4.5] (trademark) is a product incorporating an apodization filter. This product is an exchangeable lens for a single lens reflex camera using 35 mm film. In a single lens reflex camera with exchangeable lens, the focus detection is ordinarily performed by using an autofocus (AF) mechanism of a phase difference detection method (more specifically a focus detection device). However, in focus defection devices employing phase difference detection method, when a transmission rate is changed continuously at the pupil position of the image-taking lens due to an apodization filter, then an illumination distribution of an object for focus detection which is projected onto an AF sensor is not uniform, so that focus detection discrepancies occur. Consequently, this produce was a lens for manual focusing only. (The foregoing has been summarized from “SHASHIN KOUGYOU”, publ. by KK. SHASHIN KOUGYOU SHUPPANNSHA, April 1999 issue, p. 77.